In a heavy saucepan, combine the first 6 ingredients (butter, sugar, brown sugar, evaporated milk, pumpkin, pumpkin pie spice); cook mixture over medium heat.: The first sensory sign is the butter melting into a glossy pool, followed by the sugars dissolving and the pumpkin folding in to create a uniform amber mixture. You will notice a gentle simmer forming, with little bubbles rising steadily, and the aroma will shift from sweet butter to warm spices. Why this matters, the even heat helps dissolve sugar granules and prevents localized scorching, which would yield grainy fudge. Troubleshooting tip, if you see dark specks forming on the pan bottom, immediately reduce the heat and stir more vigorously to prevent burning.
Bring mixture begins to a boil, stirring constantly. Boil mixture until candy thermometer reaches soft-ball stage (234-243 degrees). It's best to use a candy thermometer but if you don't have one you can boil the mixture until it gets foamy and once it's foamy boil for an additional 4-6 minutes.: The mixture will transform, becoming frothy as steam drives moisture out, and the sound will change to a steady, higher pitched simmer. The visual cue is a thickening stream when you lift the spoon, and the scent intensifies into deep caramel notes. Hitting the soft ball stage is crucial because it determines set and chew. Troubleshooting tip, if you undercook it, the fudge will be too soft and sticky; if you overcook, it will become dry and crumbly.
Remove pan from heat; stir in chocolate chips until melted. It's important to remove it from the heat so that the chips don't get overheated and turn grainy.: Off the heat, the white chocolate chips will melt into silky ribbons when stirred, producing a glossy, pale mixture. You will feel the viscosity shift under the spoon as it smooths out, and the aroma will sweeten with cocoa butter notes. This step matters because residual heat finishes the melt without breaking the cocoa butter emulsion, which would lead to a mealy texture. Troubleshooting tip, if you see graininess forming, quickly continue stirring off heat and consider a brief warm water bath under the pan while stirring to gently re-emulsify.
Add the marshmallow cream, pecans, and vanilla and stir to combine well.: Once added, the marshmallow creme will lighten the batter, creating a pillowy sheen, while the pecans add flecks of toasted color and crunchy contrast. As you fold, listen for the soft tearing sound of the marshmallow integrating, and observe the mixture become smoother and more voluminous. The why here is texture balancing, the marshmallow prevents the fudge from being too dense and the pecans deliver a needed textural counterpoint. Troubleshooting tip, folding too aggressively can deflate some of the marshmallow airiness, so fold gently until just combined.
Pour into a buttered 13 x 9 inch baking pan; cool to room temperature.: As you pour, the mixture will settle into a glossy sheet that gradually loses heat and firms around the edges first; the center will remain slightly jiggly for a while. Visual cues include a shift from liquid gloss to a matte sheen as it cools, and the scent will soften into a baked sugar note. Cooling to room temperature ensures a clean cut with even texture throughout. Troubleshooting tip, placing it in the refrigerator too soon can cause uneven setting and condensation, leading to a sticky surface.
Cut into squares; store in the refrigerator in an air-tight container. I like eating it at room temperature.: When you cut, use a sharp knife to create clean edges, and if needed, warm the blade for smoother slices. You will notice the crumb holds together and the cross section reveals pockets of nut and marshmallow. Storing chilled extends shelf life and keeps the texture stable, while bringing squares to room temperature before eating restores the fudgy mouthfeel. Troubleshooting tip, if squares crumble, they were either undercooked or the knife was dull, so recheck the set next time and use a warmed, sharp blade for slicing.